1. Field of the Invention
The present invention is directed to transformer bobbin bodies. More particularly, the present invention is directed to a molded transformer bobbin assembly.
2. Description of the Prior Art
Transformer bobbin bodies are normally formed of plastics materials. Two primary types are used, those having axially separated winding areas and those having radially separated winding areas. Such bobbin bodies may have primary and secondary winding areas, two primary and a single secondary, or two secondary and a single primary. The primary area and secondary area are normally separated by a wall member. A common design is to provide a core tube, normally rectangular in cross-section, and a plurality of outwardly projecting flange walls axially spaced along the length of the core tube to define winding areas between the walls radially of the core tube. Such transformer bobbins are formed, as desired, with other projecting members such as mounting members, pin members, etc. Commonly, the entire transformer bobbin is formed of a single molded plastic piece. Existing winding machines are designed to accommodate such single piece coil bodies.
Some countries, such as the countries of the European common market, have adopted standards providing for a minimum creepage distance between the primary and secondary windings. Although such minimum creepage distances can be established by making the projecting walls between the winding areas of a height from the outermost coil winding equal to at least half the minimum creepage distance, whereby the entire minimum creepage distance will be established as the path up one side of the wall from the winding of the one coil thence down the other side of the wall to the winding of the other coil, such a solution is undesirable. In particular, this extended wall solution would require a greater than desired height for the overall transformer which height would be made up mostly of wasted space, i.e. the space outward from the outermost winding. Since the metal laminates of the transformer core are commonly E-shaped, having a central tang in the core tube and legs extending axially of the bobbin body immediately outwardly of the walls, such a solution would provide a greater spacing between the legs and tang than is otherwise desired.
Various attempts have been made to overcome such problems, e.g., U.S. Pat. No. 3,909,761 (Miles-Platts Limited) discloses transformer bobbins moulded from plastic providing spool portions for primary and secondary windings of the transformer which are separated by a radially extending flange. A shroud is provided in the form of a casing which at least partially encloses the windings and has an internally formed ridge which engages the edge of the flange in an interleaving manner to minimize the "tracking" of current between the windings. In other words, the radially extending flange has a double lip formed on its periphery which encloses a groove, and this groove receives the internally formed ridge of the shroud therein, when the shroud and windings are assembled.
U.S. Pat. Nos. 4,405,913 and 4,510,478 (Mid-West Transformer Company) disclose a transformer coil body providing an elongated creepage path between primary and secondary windings in the form of at least one U-shaped channel member with a portion thereof projecting between the primary and secondary windings to provide a serpentine creepage path. In particular, a hollow core tube is provided with outwardly extending end walls and an intermediate separator wall. Additional walls are provided closely spaced to and parallel to one of the end walls and the intermediate separator wall, whereby a channel or space about the core is formed between the one end wall and its associated additional wall and the intermediate separator wall and its associated additional wall. A cap member is also provided, the cap member having an inturned leg for receipt between the separating wall and the associated additional wall. The cap member leg is three sided, being substantially U-shaped so that the leg is inserted in the gap between the separating wall and the associated additional wall around three sides of the core tube. The cap is also provided with a second inturned leg which fits in a corresponding manner in the gap between the end wall and its associated additional wall.
U.S. Pat. No. 4,716,394 (Cosmo Plastics Company) discloses a transformer bobbin assembly comprising a U-shaped shroud having a pair of bobbins removably mounted therein. The bobbins include detents to indicate proper positioning with respect to each other and cooperatively provide mounting channels for receipt of an intermediate rib of the shroud and spaced leg guides extending transversely inwardly from the shroud. In particular, the transformer bobbin assembly comprises a shroud and a pair of identical bobbins, the bobbins being sized to be received within the shroud with a sliding frictional fit. The shroud includes laterally spaced legs which are joined by a bight. The shroud has a longitudinal length corresponding with that of the assembled bobbins. Each of the legs are provided with a pair of transversely inwardly projecting leg guides. The shroud also includes a transversely inwardly projecting mounting rib, which extends around the entire inner periphery of the legs and the bight. The rib is located along a plane which axially bisects the shroud for receipt of the bobbins on each side thereof.
Each bobbin has a core which extends between a mating flange and an end flange, the core being of a rectangular cross-section, a coil winding area being defined about the core between the mating flange and the end flange. The mating flange has a mating face remote from the coil winding area. The mating face includes an interior planar surface portion which surrounds the core and which engages the interior surface portion of the other bobbin upon assembly within the shroud. The mating face includes a recessed planar surface portion extending around the periphery, which, upon assembly, cooperates with the corresponding surface on the other bobbin to define a mounting channel receiving the shroud rib.
British Patent No. 1 529 136 (Pye (Electronic Products) Limited) discloses a bobbin, moulded from an electrically insulating plastic material, which has a central core portion and a pair of end flanges, the space between the inner faces of the end flanges defining a winding space for a coil. One of the end flanges is substantially rectangular and has first and second integral members extending from opposite sides thereof at right angles to the plane of the outer face of the one end flange. A third integral member joins the sides of the first and second members remote from the end flange. The one end flange and the first, second and third members defining a rectangular prismatic space adapted to receive a second bobbin inserted in a direction parallel to the one end flange. In a preferred embodiment, the second bobbin which comprises a core and a pair of end flanges, has a member extending transversely from one edge of one of the end flanges so as to provide an increased creepage between the coils.
U S. Pat. No. 3,843,946 (Original Equipment Motors, Inc.) discloses a bobbin for a motor coil constructed with a flange having a lead wire retainer extended therefrom. The retainer is in the form of a tab having spaced fingers which present pockets for capturing the lead wires. The bobbin is received within a casing or housing provided with a slot for receiving the retainer tab.
As may be readily ascertained from the above, the prior art generally utilizes a middle or inside flange which is provided with a slot, the slot receiving a rib, formed on the shroud, therein. This approach requires a greater than normal wall thickness, thus, decreasing the available winding area on a finite-sized core; and this approach requires more complex dies for molding, creating an increased tooling cost. Accordingly, a need exists for an improved construction of a transformer bobbin.